Laser time of arrival probes have been used in industry to measure vibratory deflections of rotating parts to support design validation. The probes are a nonintrusive option that may be used instead of traditional rotating strain gauges. While such probes have been shown to be helpful to the determination of low order/frequency fundamental vibratory modes where expected deflections are relatively large, the use of these probes has been significantly less beneficial to the measurement and determination of high order/frequency modes of vibration typically associated with very low deflection to stress ratios, and therefore very low expected deflections resulting in low signal to noise ratios.
In contrast to measuring low order/frequency modes, successfully measuring high order/frequency modes requires a large number of probes located at different circumferential locations proximal to the rotating part. Such probes should be located in regions where maximum modal deflections occur. If the selected measurement location on the part is not at a local maximum, the measured deflection may be hidden within the noise and the measurement becomes unusable.
High order/frequency modes may also be sensitive to manufacturing variations which cause the location of maximum vibratory deflection to vary from part to part, for example, blade to blade variation within a rotor stage. A blade is an example of a part/subcomponent where a set of such parts/subcomponents when assembled or integrally manufactured as a rotor make up a larger component referred to as a rotor stage. Thus, it may be difficult to select a single repeatable measurement location for all blades within a rotor stage. If the location of the maximum local deflection is not consistent on each blade, it may be very difficult to determine the stress from a measured deflection of the part/subcomponent. To solve these problems a very large and impractical number of probes may be required to increase the probability of measuring the local maximum deflection.
Furthermore, when more than one mode of vibration is being measured often there is a requirement for the probes to be located proximal to multiple locations on the part. In addition, high order/frequency modes may be closely spaced in the frequency domain, and therefore, it may be difficult to correctly identify the mode of vibration, especially when frequency mistuning causes closely spaced modes to simultaneously occur. If the mode is incorrectly identified, the stress to deflection ratios used to estimate stress may not be useful.